The Modular Multilevel Converter (M2C) is a voltage source converter topology which may be used for AC/DC and DC/AC conversion in high-voltage high-power applications. Each phase leg of the typical Modular Multilevel converter has a number of series connected converter cells and two inductors. The converter cells, termed submodules, are generally controlled half bridges equipped with DC capacitors. Each chain of series connected submodules is referred to as one arm. The arm connecting the AC terminal to the positive DC terminal is referred to as the upper arm. Similarly, the arm connecting the AC terminal to the negative DC pole is referred to as the lower arm. The converter is controlled in such a way that the stored energy in the capacitors remains constant over time. This means that the capacitors act as voltage sources that can be either inserted or bypassed in the arms. The voltages at the AC and DC terminals can then be controlled by varying the number of inserted submodules in each arm.
In order to ensure that the stored energy in each submodule remains essentially constant over time, a capacitor voltage balancing strategy may be used. One possible solution is to use a conventional multilevel pulsewidth modulation (PWM) method, such as phase shifted carriers, and then control the energy levels by means of a feedback controller for each submodule. An example of this method is presented in the article “Control and Experiment of Pulsewidth-Modulated Modular Multilevel Converters” by Makoto Hagiwara and Hirofumi Akagi, published in IEEE Transactions on Power Electronics, vol. 24, No. 7, July 2009. The stored energy can also be kept constant over time by rotating the pulse pattern between the submodules as described in the article “A New Modulation Method for the Modular Multilevel Converter Allowing Fundamental Switching Frequency,” by lives, K.; Antonopoulos, A.; Norrga, S.; Nee, H., published in IEEE Transactions on Power Electronics, vol. 27, No. 8, August 2012. Yet another possible solution is to use a sorting algorithm that selects which submodule to insert or bypass based on the sign of the current as described in the paper “An Innovative Modular Multilevel Converter Topoology Suitable for a Wide Power Range” by A. Lesnicar and R. Marquardt, published in 2003 IEEE Bologna PowerTech Conference Proceedings, Jun. 23-26, 2003, Bologna, Italy. That is, if one submodule is to be inserted and the current is positive, the submodule with the lowest voltage is inserted. If the current is negative, the submodule with the highest voltage is inserted. Similarly, if one submodule is to be bypassed, the submodule with the highest voltage is bypassed if the current is positive. If the current is negative, the submodule with the lowest voltage is bypassed.
However simulations of the capacitor voltage ripple in a modular multilevel converter using the above mentioned sorting algorithm have shown that although the capacitor voltages remains constant over time, the maximum deviation of the voltage ripple can be considerable compared to the case when the charge is distributed evenly between the submodules. Alternative methods for capacitor voltage balancing which allows for reduced voltage ripple would therefore be of interest.